Indian

Small quantities of silicon carbide grit is available from lapidary suppliers and inj a wide range of grits.

You need continuous cast bar, most of the usual suppliers sell it. M Machine, Noggin end, College etc.

Its a shame you are avoiding screw cutting as it is a necessity on most model engines and a good skill to learn.

As I said before if you turn the outside taper and a d bit blank at the same time they will fit perfectly, same goes for your Venturi and if the material being cut is alu no need for fancy hardening if you are only going to use it once or twice.

Your lathe should be able to turn short tapers and if you leave your compound set at the taper you can then machine another taper on a piece of silver steel to make a D bit ensuring the two pieces will fit together perfectly. As for allowing for free play between the pulley and the crankcase its a simple matter to skim the pulley to get some. The taper on the crank makes it less likely to give stress risers and therefore failures.